Support Sass 3.5+ (while dropping support for Sass < 3.3)

[agrobbin]

Sass 3.5 introduced support for bracketed lists (sass/sass@be02da0), which changed the method signature of Sass::Script::Value::List#initialize to accept the separator as a keyword argument rather than as the second positional argument.

This changes our implementation to support both the pre- and post-3.5 world.

While we're at it, it's time to drop support for Sass < 3.3:

• 3.1.x was last released on 2012-08-10 (3.1.21)
• 3.2.x was last released on 2014-04-05 (3.2.19)

3.3.x was also last released in 2014 (2014-08-01 as 3.3.14), but supporting the last 3 major versions (Sass only started following Semver from 3.5 onwards) seems like a reasonable amount.

[agrobbin]

Tests are failing because Ruby 2.0 and Nokogiri (a development dependency from Rails) don't get along, but we're about to drop support for Ruby 2.0 (and 2.1 to boot) too, so the failures are OK.